`stream` has to be keyword-only argument in `to_device` method

[antonwolfy]

According to python array API notation:

array.to_device(device: device, /, *, stream: int | Any | None = None) → array

but it is not followed by dpctl implementation, where:

def to_device(self, target_device, stream=None):

Also it seems stream argument is not covered by any test in dpctl.